The present invention relates to a sound attenuating box, particularly for the attenuation of sound waves which propagate within a fluid flowing in channels such as venting ducts. Such a sound attenuation box of the type to which the invention pertains, includes (a) at least one closed chamber establishing a cavity which is bounded on one side by an outer plate which can be stimulated to undergo vibrations in the audible range; and (b) at least one inner plate is arranged in the cavity of the chamber, running essentially parallel to the aforementioned outer plate, and partioning the cavity into two hollow chambers, which, however, are interconnected through a passage aperture traversing the internal plate. This passage aperture together with that partial chamber being separated by the inner from the outer plate, will constitute a so-called Helmholtz resonator that is tuned to vibrations in the audible range.
Industrial as well as home equipment provides for the transportation of air, exhaust fumes or the like through flow channels, containers, as well as openings. Such flow channels constitute frequently a very undesired transmission path for sound in air between the rooms interconnected by these flow channels. In particular, flow channels will transmit the noise produced by the flow generator itself, for example, a blower into otherwise closed rooms.
A rather effective sound attenuation can be obtained in such cases, in that the flow channels are covered with a sound absorbing lining. Upon utilizing a homogeneous lining of this type, the sound attenuation obtained therewith is proportional to the degree of absorption by the lining and the length of the flow channel as well as to the ratio between the circumference and the free cross-sectional areas of the flow channel. If technically realizable, the ratio between the periphery and the cross-sectional area of the free flow channel, is increased as much as possible through the insertion of additional sound absorbing partitions into the flow channel. Moreover, through this feature, the effective flow channel width is favorably reduced, because this width should always be significantly smaller than the wave length of the sound. The pressure drop produced through this kind of sound attenuation, however, must be kept as low as possible because otherwise a higher blower power has to be compensated, and that, in turn, entails a higher sound emission. These sound attenuation devices must, therefore, not exceed a particular thickness and should offer very little resistance to the flow.
In order to reduce to a minimum the friction losses which occur on the surface of the sound absorbing layer, as well as on the sound attenuating elements, it is desirable to make these surfaces completely planar, homogeneous, and smooth, i.e. without any apertures, steps, jumps, or the like.
Whenever precipitation deposits from the flow medium are to be expected, such as it may occur in air, gas, or vapor, then the sound absorbing element should not accept these deposits in any manner whatever, because deposits on the outside would not only increase the friction losses but, in the case of the usual sound absorber, their acoustic effectiveness would be considerably reduced. Even more important, however, is that the penetration of material from the flow medium into the interior of the sound absorber has to be avoided because these materials, when penetrating the absorber, will accumulate therein and, therefore, unfavorably interfer with the effectiveness of the sound absorber, or there may even be chemical reactions or other undesirable interactions. Hence, such a penetration of material should be avoided under all circumstances. For these reasons, as well as other important reasons having to do with safety technology such as fire proofing or hygenic-health aspects, it is desirable to provide an all around, particularly gas and water proof sealing of the sound absorbing lining elements as between the flow channels and the adjoining rooms.
In order to insure a long use life and a lasting sealing of the sound attenutating elements, at least their outer skin or shell, should be chemically, as well as mechanically, resistant as much as possible. The sound attenuating element should not just stand in the flow of fluid in the flow channel as a kind of a sensitive intruder, but should, so to speak, harmonically merge in the construction of the entire flow system. Therefore, the material for the sound attenuating elements should as much as possible be matched to the material used for the flow channel walls. Particularly a construction made of sheet metal would be highly desirable. Moreover, the sound attenuating elements should be sufficiently stable so that even in the case of a long transport from the manufacturer to the final destination, they will survive even rather rough conditions of handling, and after installation, even in case of loads by the flow or in case of shaking through the flow producing and conveying device, the same attenuating should withstand such load. On the other hand, their weight should be as low as possible, particularly in view of the required assembly, as well as for reasons of statics.
In many instances, therefore, a sound attenuation construction would be particularly adantageous if the sound absorbing material were not arranged within a frame of stably constructed frame that has to be held in addition and protected against damage, by the flow of material, but wherein the sound absorbing parts individually or in combination, can take over load bearing and stiffening functions, i.e. it would be of advantage if the attenutation carrying parts of the sound attenuation structure could also contribute to the sound absorption itself. The known sound attenuators do not even come close to fulfilling all of these requirements.
In addition, for example, in case of venting equipment, there is a typical situation, which, in view of the sound absorbing range is of particular importance. This condition is to be seen in that high and medium frequencies will be attenuated in long flow channels, even without or with very simple sound absorbing features, as applied to the walls of the flow channels. However, lower frequencies, i.e. frequencies below 500 cps pose frequently difficulties. In addition, the noise spectrum of blowers has typically already at the point of sound emission, a characteristic that drops with frequency. The situation is quite similar generally with aerodynamically produced noise in the flow channels which occur also mostly at lower frequencies. Even if the so-called A-evaluation is carried out for matching to the ear, very often frequencies in the range of 250 cycle/sec, remain dominant at the point of emission.
Actually, one should expect that under these circumstances, one should use sound attenuators which are tuned preferably to the low frequencies. Instead, however, one finds predominantly sound attenuation which have their largest effectiveness well above the dominating frequency range encountered. The reason for this is, simply stated, that the porous of fibrous material used for the sound absorber, are constructively similar, quite proven, and can safely be installed. Even though the attenuation of low frequencies by means of porous sound absorbers is made possibly only whith very thick sound attenuating elements. These advantages have been put up with in practice, but particularly the disadvantage of a large volume of the resulting construction and, therefore, undesirable high pressure losses.
In accordance with the state of the art, sound attenuators are generally conceived for a universal application in sound attenuation covering a large variety of noise sources, with very different composition as far as the participating sound frequencies are concerned. For these applications which are not specifically tuned to a specific noise spectrum, a lining is preferably used in accordance with the state of the art which in as wide as possible a frequency range, guarantees a rather high and constant sound attenuation. However, as was outlined above, there is a large area, particularly in the field of venting in which the conventional sound absorbers simply have their maximum effectiveness in a frequency range quite far from the desirable frequency range, and are thus used only at marginal effectiveness. That, in turn, has lead to the fact that sound attenuators for this application are in rather senseless manner not rated and selected or designed in accordance with the attenuation maximum they can achieve, but instead, only as to their attenuation at a 250 cps frequency.
The sound absorbing construction part which is made without the utilization of porous sound absorbing material and possesses the features (a) and (b) mentioned above is, for example, disclosed by the applicant-assignee in German patent application No. P 34 12 432, filed Apr. 3, 1984. This sound absorbing construction element is made of individual twin cup-shaped sound absorbing elements which have two nested cup elements, whereby the cup opening of the inner one of the elements is covered by means of a, preferably planar foil, and is particularly and to a substantial degree closed in a sound proofing fashion. Between the side walls of the two nested cup elements a gap is formed which establishes the sound absorbing air or gas layer for bending mode vibrations in the audible frequency range and involving the side wall of the outer and/or the inner cup element. The inner cup element is not as high as the outer cup element, and the bottom of the inner cup element is provided with a neck-like passage opening, having a cross-section and a length such that the resonating frequency of the neck-shaped passage opening corresponds to the Helmholtz resonator frequency in the audible range, and involving the volume of the inner cup element as well as the volume bounded by the outer cup element and by the bottom of the inner cup element. This sound absorbing construction element combines a broad band sound absorption characteristics with a sound attenuation device which is particularly attuned to low frequencies as they occur on account of the blades of a blower, so that it seems to be quite suitable for attenuating sound waves as they propagate in the flow channels, for example, in case of venting channels. However, this device is of rather complex construction so that for reasons of cost, it will not be used in all points where it is usable. Moreover, this attenuator is not provided with completely smooth boundary surfaces so that for reasons of possible depositing of material from the flow limits its usefulness.